Gaseous chlorine dioxide in low concentrations (i.e. up to 1,000 ppm) has long been recognized as useful for the treatment of odors and microbes. Its use is particularly advantageous where microbes and/or organic odorants are sought to be are sought to be controlled on and around foodstuffs, as chlorine dioxide functions without the undesirable side products such as chloramine or chlorinated organic compounds that can be produced when elemental chlorine is utilized for the same or similar purposes. For example, if a low concentration of chlorine dioxide gas can be maintained in contact with fresh produce for several days during shipping from the farm to the local retailer, the rate of spoilage of the products can be decreased. In addition, chlorine dioxide gas is also generally considered to be safe for human contact at the low concentrations that are effective for deodorization and most anti-microbial applications.
Chlorine dioxide gas can be toxic to humans at concentrations greater than 1,000 ppm and it can be explosive at concentrations above about 0.1 atmosphere. Therefore, chlorine dioxide gas is not manufactured and shipped under pressure like other industrial gases, and conventional methods of on-site manufacture require not only expensive generation equipment but also high levels of operator skill to avoid generating dangerously high concentrations. These problems have substantially limited the use of chlorine dioxide to large commercial applications, such as water treatment and poultry processing, where the consumption of chlorine dioxide is sufficiently large that it can justify the capital and operating costs of expensive equipment and skilled operators for on-site manufacture.
Commercially, chlorine dioxide is produced from a variety of aqueous solutions of chlorine-containing salts, for example see U.S. Pat. No. 5,009,875.
Attempts have also been made to produce chlorine dioxide using mixtures of solid reagents. Generally, the prior art has focused on three systems for chlorine dioxide production using solid reagents. One system employs a solid mixture of a metal chlorite and an acid in a liquid, aqueous environment. A second system combines a metal chlorite and an acid where chlorine dioxide gas is released under dry conditions. A third system employs the combination of a metal chlorite and a solid organic acid anhydride to generate a highly concentrated flow of chlorine dioxide which must be diluted with a constantly flowing stream of inert gas.
Each of these solid reagent systems is disadvantageous for any one or more of the following reasons:                a) upon mixing there is normally a sudden, highly concentrated stream of chlorine dioxide generated;        b) the mixture of reactants produces chlorine dioxide gas under dry conditions thereby reducing the shelf life of the reactants; and        c) an inert gas stream must be used to reduce the concentration of chlorine dioxide gas in the atmosphere.        
For example: U.S. Pat. No. 2,022,262 discloses the use of chlorine dioxide in aqueous solution in a stain removing process wherein the chlorine dioxide is produced upon acidification of an aqueous solution of alkali metal or alkaline earth metal chlorite salts (i.e. chlorites) with oxalic acid.
U.S. Pat. No. 2,071,091 discloses that chlorous acid which is produced upon acidification of solutions of alkali metal and alkaline earth metal chlorite salts is an effective fungicide and bactericide. This patent discloses solid compositions of metal chlorites and solid acids that will produce chlorine dioxide when dissolved in water. However, the materials of the '091 patent are useful only in “wet” applications where liquid water is available and where contacting a material to be treated with chlorine dioxide dissolved in liquid water is acceptable.
U.S. Pat. No. 2,071,094 discloses deodorizing compositions in the form of dry briquettes comprising a dry mixture of a soluble chlorite, an acidifying agent, and a filler of a lower solubility so that disintegration of a briquette is inhibited in the presence of liquid water. Generation of chlorine dioxide begins as the briquette dissolves in water. Such materials are subject to the same use limitations as those of the '091 Patent.
U.S. Pat. No. 2,482,891 discloses a material comprising a solid organic acid anhydride and an alkali metal or alkaline earth metal chlorite salt which is stabilized by the addition of a desiccant material. The combined solid material is described as evolving chlorine dioxide on contact with water. Example 1 describes the production of chlorine dioxide by contacting a mixture of sodium chlorite, phthalic anhydride and sodium monoxide with water vapor. It is not clear from the example whether or not the solid mixture was already in contact with liquid water. The resultant exit gas in this example contains a high concentration of chlorine dioxide gas. Also, the organic acid anhydride is potentially explosive in combination with the chlorite salt, as well as being a relatively expensive constituent. Therefore, this material has not been commercially successful.
U.S. Pat. No. 3,591,515 discloses solid pulverulent compositions comprising solid carriers having impregnated thereon stabilized solutions of chlorine dioxide or chlorites. When the solution-impregnated compositions are contacted with solid acids they release chlorine dioxide gas. Such materials are sold commercially today under the trade names OSTOBON® and ABSCENT® (by International Dioxide Inc., Clark, N.J.), but their commercial acceptance has been limited because they either prematurely release small amounts of chloride dioxide through the packaging on store shelves, or they require relatively complicated mixing of two ingredients by the user at the point of application.
U.S. Pat. No. 4,585,482 discloses a long-acting biocidal composition comprising a chlorite and an organic acid such that the pH of the composition is <7. Such compositions release chlorine dioxide in the presence of liquid water. This patent also discloses methods for producing dry microcapsules of such compositions with water having polymer shells such that the resultant dry materials release chlorine dioxide.
U.S. Pat. No. 4,547,381 discloses dry compositions for the sustained controlled release of gaseous chlorine dioxide comprising a dry inert diluent, a chlorite salt, and a dry agent capable of reacting with a chlorite in a dry state to produce chlorine dioxide gas. Such materials have not achieved substantial commercial success because they begin to release chlorine dioxide gas immediately upon formulation and, therefore, they must be mixed and utilized over a short time period.
U.S. Pat. No. 5,360,609 discloses the incorporation of a chlorine dioxide generating compound into a polymer or oligomer film which is then coated onto a substrate. The chlorite constituent is dissolved in a hydrogen bonded phase containing a monomeric or polymeric amide or alcohol. The hydrogen bonded phase is then mixed with an incompatible apolar phase containing an acid anhydride. Chlorine dioxide gas is released by direct reaction of the acid anhydride with the chlorite anion across the phase boundary. However, the process described in the '609 patent employs relatively expensive materials and the reaction is potentially explosive due to the proximity of the strongly oxidizing metal chlorite with the carbonaceous polymers.
U.S. Pat. No. 5,567,405 discloses the generation of chlorine dioxide gas from mixed beds of zeolite crystals, where the first bed comprises a zeolite that has been impregnated with an aqueous solution of sodium chlorite and the second bed comprises a zeolite that has been impregnated with phosphoric, citric, or acetic acid. Chlorine dioxide gas is released when acid migrates from the second bed and contacts chlorite on the first bed. The first and second beds may be physically mixed together. The process disclosed in the '405 patent requires expensive equipment and results in a product having a relatively short shelf-life.
It would therefore be a significant advance in the art of generating chlorine dioxide gas for commercial applications to have a method, composition and system in which the chlorine dioxide gas is generated under controlled conditions at low concentrations. It would be a further advance in the art to provide a method, composition and system in which the reactants do not generate chlorine dioxide gas in the absence of water but do provide a controlled sustained release of chlorine dioxide gas in the presence of water vapor. As a result, the composition of the present invention can be prepared in advance and stored under dry conditions without the premature release of chlorine dioxide gas. In this manner the need for skilled personnel to prepare the mixture on-site is avoided and shelf-life is enhanced.